The Metal Gear REX is a Metal Gear model designed by Hal "Otacon" Emmerich as a secret project for the U.S. Department of Defense.
Like other Metal Gear units, REX is a bipedal walking tank with the ability to launch nuclear weapons. When Liquid Snake leads the members of FOXHOUND in the revolt on Shadow Moses Island, he threatens to use REX to launch a nuclear warhead unless his demands are met.